diff --git a/includes/cpt/class-pr-core-topic-taxonomy.php b/includes/cpt/class-pr-core-topic-taxonomy.php
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+++ b/includes/cpt/class-pr-core-topic-taxonomy.php
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+<?php
+declare(strict_types=1);
+
+/**
+ * Registers the peptide_topic taxonomy for linking blog posts to peptides.
+ *
+ * What: Defines the `peptide_topic` taxonomy attached to the post CPT.
+ *       Non-hierarchical, uses peptide slugs as terms (e.g., 'bpc-157', 'tb-500').
+ *       Enables the Related Articles feature for blog posts tagged with a peptide topic.
+ * Who calls it: PR_Core::init() on plugins_loaded.
+ * Dependencies: None.
+ *
+ * Ownership: As of v0.3.0, PR Core owns `peptide_topic` taxonomy.
+ * Registration is guarded by `taxonomy_exists()` so deploy order does not matter.
+ *
+ * @see ARCHITECTURE.md — Taxonomy specification.
+ * @see CONVENTIONS.md — Taxonomy ownership rule.
+ */
+class PR_Core_Topic_Taxonomy {
+
+	/** @var string Taxonomy: topic (links blog posts to peptides by slug). */
+	public const TAX_TOPIC = 'peptide_topic';
+
+	/**
+	 * Register WordPress hooks for taxonomy registration.
+	 *
+	 * @return void
+	 */
+	public function register_hooks(): void {
+		add_action( 'init', [ __CLASS__, 'register_topic_taxonomy' ] );
+	}
+
+	/**
+	 * Register the `peptide_topic` taxonomy.
+	 *
+	 * Guarded with `taxonomy_exists()`: if another plugin has already
+	 * registered the `peptide_topic` taxonomy, this call no-ops.
+	 *
+	 * Side effects: registers taxonomy with WordPress.
+	 *
+	 * @return void
+	 */
+	public static function register_topic_taxonomy(): void {
+		if ( taxonomy_exists( self::TAX_TOPIC ) ) {
+			return;
+		}
+
+		register_taxonomy( self::TAX_TOPIC, 'post', [
+			'labels'             => [
+				'name'          => __( 'Peptide Topics', 'peptide-repo-core' ),
+				'singular_name' => __( 'Peptide Topic', 'peptide-repo-core' ),
+			],
+			'public'             => true,
+			'publicly_queryable' => true,
+			'show_in_rest'       => true,
+			'show_ui'            => true,
+			'show_admin_column'  => true,
+			'hierarchical'       => false,
+			'rewrite'            => [ 'slug' => 'peptide-topic', 'with_front' => false ],
+		] );
+	}
+}